Make DuckDB connection a singleton

[JelteF]

In #174 we made the DuckDB database a singleton, but a new DuckDB connection was still being created for every query. This means that any connection state, such as SET commands or temporary tables don't survive a single query, which is pretty confusing. This addresses that by also making the duckdb::Connection a singleton, this way a Postgres connection and a DuckDB connection are linked 1-to-1.